If my memory isn't just making fun of me, I THINK there used to be a weapon set voter a few years back. I may be wrong though. I do recall seeing a voter to change gravity, enable turbo mode and other stuff.
I'm pretty sure a gamemode vote would be possible since it only has to execute a single command for the next gamemode to be changed, and I wouldn't be surprised if someone made one in the past.
I'd wait for additional replies, I've never hosted a GE:S server myself but I'm pretty sure you could find what you're looking for.